Eddy Convariance

This highly practical handbook is an exhaustive treatment of eddy covariance measurement that will be of keen interest to scientists who are not necessarily specialists in micrometeorology. The chapters cover measuring fluxes using eddy covariance technique, from the tower installation and system dimensioning to data collection, correction and analysis. With a state-of-the-art perspective, the authors examine the latest techniques and address the most up-to-date methods for data processing and quality control. The chapters provide answers to data treatment problems including data filtering, footprint analysis, data gap filling, uncertainty evaluation, and flux separation, among others. The authors cover the application of measurement techniques in different ecosystems such as forest, crops, grassland, wetland, lakes and rivers, and urban areas, highlighting peculiarities, specific practices and methods to be considered. The book also covers what to do when you have all your data, summarizing the objectives of a database as well as using case studies of the CarboEurope and FLUXNET databases to demonstrate the way they should be maintained and managed. Policies for data use, exchange and publication are also discussed and proposed. This one compendium is a valuable source of information on eddy covariance measurement that allows readers to make rational and relevant choices in positioning, dimensioning, installing and maintaining an eddy covariance site; collecting, treating, correcting and analyzing eddy covariance data; and scaling up eddy flux measurements to annual scale and evaluating their uncertainty. Eddy Covariance Preface Contents Contributors Chapter 1: The Eddy Covariance Method 1.1 History 1.2 Preliminaries 1.2.1 Context of Eddy Covariance Measurements 1.2.2 Reynolds Decomposition 1.2.3 Scalar Definition 1.3 One Point Conservation Equations 1.3.1 Dry Air Mass Conservation (Continuity) Equation 1.3.2 Momentum Conservation Equation 1.3.3 Scalar Conservation Equation 1.3.4 Enthalpy Equation 1.4 Integrated Relations 1.4.1 Dry Air Budget Equation 1.4.2 Scalar Budget Equation (Generalized Eddy Covariance Method) 1.5 Spectral Analysis 1.5.1 Spectral Analysis of Turbulence 1.5.2 Spectral Analysis of Atmospheric Turbulence 1.5.3 Sensor Filtering 1.5.4 Impacts of Measurement Height and Wind Velocity References Chapter 2: Measurement, Tower, and Site Design Considerations 2.1 Introduction 2.2 Tower Considerations 2.2.1 Theoretical Considerations for Tower Design 2.2.1.1 Diverse Ecosystems and Environments 2.2.1.2 Physical Effects on Surrounding Flows Due to the Presence of Tower Structure 2.2.1.3 Size of Horizontal Supporting Boom 2.2.1.4 Tower Deflection and Oscillations 2.2.1.5 Recirculation Zone at the Opening in a Tall Canopy 2.2.2 Tower Design and Science Requirements 2.2.2.1 Tower Location Requirements 2.2.2.2 Tower Structure Requirements 2.2.2.3 Tower Height Requirements 2.2.2.4 Tower Size Requirements 2.2.2.5 Instrument Orientation Requirements 2.2.2.6 Tower Installation and Site Impact Requirements 2.3 Sonic Anemometer 2.3.1 General Principles 2.3.2 Problems and Corrections 2.3.3 Requirements for Sonic Choice, Positioning, and Use 2.4 Eddy CO2/H2O Analyzer 2.4.1 General Description 2.4.2 Closed-Path System 2.4.2.1 Absolute and Differential Mode 2.4.2.2 Tubing Requirements for Closed-Path Sensors 2.4.2.3 Calibration for CO2 2.4.2.4 Water Vapor Calibration 2.4.3 Open-Path Systems 2.4.3.1 Installation and Maintenance 2.4.3.2 Calibration 2.4.4 Open and Closed Path Advantages and Disadvantages 2.4.5 Narrow-Band Spectroscopic CO2 Sensors 2.5 Profile Measurement 2.5.1 Requirements for Measurement Levels 2.5.2 Requirements for Profile Mixing Ratio Measurement References Chapter 3: Data Acquisition and Flux Calculations 3.1 Data Transfer and Acquisition 3.2 Flux Calculation from Raw Data 3.2.1 Signal Transformation in Meteorological Units 3.2.1.1 Wind Components and Speed of Sound from the Sonic Anemometer 3.2.1.2 Concentration from a Gas Analyzer 3.2.2 Quality Control of Raw Data 3.2.3 Variance and Covariance Computation 3.2.3.1 Mean and Fluctuation Computations 3.2.3.2 Time Lag Determination 3.2.4 Coordinate Rotation 3.2.4.1 Requirements for the Choice of the Coordinate Frame and Its Orientation 3.2.4.2 Coordinate Transformation Equations 3.2.4.3 Determination of Rotation Angles 3.3 Flux Determination 3.3.1 Momentum Flux 3.3.2 Buoyancy Flux and Sensible Heat Flux 3.3.3 Latent Heat Flux and Other Trace Gas Fluxes 3.3.4 Derivation of Additional Parameters References Chapter 4: Corrections and Data Quality Control 4.1 Flux Data Correction 4.1.1 Corrections Already Included into the Raw Data Analysis (Chap. 3) 4.1.2 Conversion of Buoyancy Flux to Sensible Heat Flux (SND-correction) 4.1.3 Spectral Corrections 4.1.3.1 Introduction 4.1.3.2 High-Frequency Loss Corrections 4.1.3.3 Low-Cut Frequency 4.1.4 WPL Corrections 4.1.4.1 Introduction 4.1.4.2 Open-Path Systems 4.1.4.3 WPL and Imperfect Instrumentation 4.1.4.4 Closed-Path Systems 4.1.5 Sensor-Specific Corrections 4.1.5.1 Flow Distortion Correction of Sonic Anemometers 4.1.5.2 Correction Due to Sensor Head Heating of the Open-Path Gas Analyzer LiCor 7500 4.1.5.3 Corrections to the Krypton Hygrometer KH20 4.1.5.4 Corrections for CH4 and N2O Analyzers 4.1.6 Nonrecommended Corrections 4.1.7 Overall Data Corrections 4.2 Effect of the Unclosed Energy Balance 4.2.1 Reasons for the Unclosed Energy Balance 4.2.2 Correction of the Unclosed Energy Balance 4.3 Data Quality Analysis 4.3.1 Quality Control of Eddy Covariance Measurements 4.3.2 Tests on Fulfilment of Theoretical Requirements 4.3.2.1 Steady State Tests 4.3.2.2 Test on Developed Turbulent Conditions 4.3.3 Overall Quality Flag System 4.4 Accuracy of Turbulent Fluxes After Correction and Quality Control 4.5 Overview of Available Correction Software References Chapter 5: Nighttime Flux Correction 5.1 Introduction 5.1.1 History 5.1.2 Signs Substantiating the Night Flux Error 5.1.2.1 Comparison with Bottom Up Approaches 5.1.2.2 Sensitivity of Flux to Friction Velocity 5.1.3 The Causes of the Problem 5.2 Is This Problem Really Important? 5.2.1 In Which Case Should the Night Flux Error Be Corrected? 5.2.2 What Is the Role of Storage in This Error? 5.2.3 What Is the Impact of Night Flux Error on Long-Term Carbon Sequestration Estimates? 5.2.4 What Is the Impact of the Night Flux Error on Functional Relationships? 5.2.5 What Is the Impact of the Night Flux Error on Other Fluxes? 5.3 How to Implement the Filtering Procedure? 5.3.1 General Principle 5.3.2 Choice of the Selection Criterion 5.3.3 Filtering Implementation 5.3.4 Evaluation 5.4 Correction Procedures 5.4.1 Filtering+Gap Filling 5.4.2 The ACMB Procedure 5.4.2.1 History 5.4.2.2 Procedure 5.4.2.3 Evaluation References Chapter 6: Data Gap Filling 6.1 Introduction 6.2 Gap Filling: Why and When Is It Needed? 6.3 Gap-Filling Methods 6.3.1 Meteorological Data Gap Filling 6.3.2 General Rules and Strategies (Long Gaps) 6.3.2.1 Sites with Management and Disturbances 6.3.3 Methods Description 6.3.3.1 Mean Diurnal Variation 6.3.3.2 Look-Up Tables 6.3.3.3 Artificial Neural Networks 6.3.3.4 Nonlinear Regressions 6.3.3.5 Process Models 6.4 Uncertainty and Quality Flags 6.5 Final Remarks References Chapter 7: Uncertainty Quantification 7.1 Introduction 7.1.1 Definitions 7.1.2 Types of Errors 7.1.3 Characterizing Uncertainty 7.1.4 Objectives 7.2 Random Errors in Flux Measurements 7.2.1 Turbulence Sampling Error 7.2.2 Instrument Errors 7.2.3 Footprint Variability 7.2.4 Quantifying the Total Random Uncertainty 7.2.5 Overall Patterns of the Random Uncertainty 7.2.6 Random Uncertainties at Longer Time Scales 7.3 Systematic Errors in Flux Measurements 7.3.1 Systematic Errors Resulting from Unmet Assumptions and Methodological Challenges 7.3.2 Systematic Errors Resulting from Instrument Calibration and Design 7.3.2.1 Calibration Uncertainties 7.3.2.2 Spikes 7.3.2.3 Sonic Anemometer Errors 7.3.2.4 Infrared Gas Analyzer Errors 7.3.2.5 High-Frequency Losses 7.3.2.6 Density Fluctuations 7.3.2.7 Instrument Surface Heat Exchange 7.3.3 Systematic Errors Associated with Data Processing 7.3.3.1 Detrending and High-Pass Filtering 7.3.3.2 Coordinate Rotation 7.3.3.3 Gap Filling 7.3.3.4 Flux Partitioning 7.4 Closing Ecosystem Carbon Budgets 7.5 Conclusion References Chapter 8: Footprint Analysis 8.1 Concept of Footprint 8.2 Footprint Models for Atmospheric Boundary Layer 8.2.1 Analytical Footprint Models 8.2.2 Lagrangian Stochastic Approach 8.2.3 Forward and Backward Approach by LS Models 8.2.4 Footprints for Atmospheric Boundary Layer 8.2.5 Large-Eddy Simulations for ABL 8.3 Footprint Models for High Vegetation 8.3.1 Footprints for Forest Canopy 8.3.2 Footprint Dependence on Sensor and Source Heights 8.3.3 Influence of Higher-Order Moments 8.4 Complicated Landscapes and Inhomogeneous Canopies 8.4.1 Closure Model Approach 8.4.2 Model Validation 8.4.3 Footprint Estimation by Closure Models 8.4.4 Footprints over Complex Terrain 8.4.5 Modeling over Urban Areas 8.5 Quality Assessment Using Footprint Models 8.5.1 Quality Assessment Methodology 8.5.2 Site Evaluation with Analytical and LS Footprint Models 8.5.3 Applicability and Limitations 8.6 Validation of Footprint Models References Chapter 9: Partitioning of Net Fluxes 9.1 Motivation 9.2 Definitions 9.3 Standard Methods 9.3.1 Overview 9.3.2 Nighttime Data-Based Methods 9.3.2.1 Model Formulation: Temperature – Measurements 9.3.2.2 Reco Model Formulation 9.3.2.3 Challenges: Additional Drivers of Respiration 9.3.2.4 Challenges: Photosynthesis – Respiration Coupling and Within-Ecosystem Transport 9.3.3 Daytime Data-Based Methods 9.3.3.1 Model Formulation: The NEE Light Response 9.3.3.2 Challenges: Additional Drivers and the FLUXNET Database Approach 9.3.3.3 Unresolved Issues and Future Work 9.4 Additional Considerations and New Approaches 9.4.1 Oscillatory Patterns 9.4.2 Model Parameterization 9.4.3 Flux Partitioning Using High-Frequency Data 9.4.4 Flux Partitioning Using Stable Isotopes 9.4.5 Chamber-Based Approaches 9.4.6 Partitioning Water Vapor Fluxes 9.5 Recommendations References Chapter 10: Disjunct Eddy Covariance Method 10.1 Introduction 10.2 Theory 10.2.1 Sample Interval 10.2.2 Response Time 10.2.3 Definition of DEC 10.3 Practical Applications of DEC 10.3.1 DEC by Grab Sampling 10.3.2 DEC by Mass Scanning 10.3.3 Use of DEC to Reduce the Burden on Data Transfer and Storage 10.4 DEC in Spectral Space 10.5 Uncertainty Due to DEC 10.6 On the History of the DEC Approach References Chapter 11: Eddy Covariance Measurements over Forests 11.1 Introduction 11.2 Flux Computation, Selection, and Dependence 11.2.1 Correction for High Frequency Losses 11.2.2 Rotation Method 11.2.3 Friction Velocity Threshold 11.2.4 Selection Based on Footprint 11.3 Additional Measurements 11.3.1 Vertical Profile of Concentration in Canopy Air 11.3.2 Leaf Area Index 11.3.3 Biomass Estimates 11.3.4 Sap Flow 11.3.5 Extractable Soil Water, Throughfall, and Stem Flow 11.3.6 Heat Storage 11.4 Impact of Ecosystem Management and Manipulation References Chapter 12: Eddy Covariance Measurements over Crops 12.1 Introduction 12.2 Measurement System 12.2.1 Choice of the Site and Communication with the Farmer 12.2.2 Flux Tower and Meteorological Station Configuration 12.2.3 Measurement Height 12.2.4 Maintenance 12.3 Flux Calculation 12.4 Flux Corrections 12.4.1 Storage Term 12.4.2 Nighttime Flux Data Screening 12.5 Data Gap Filling and Footprint Evaluation 12.6 Cumulated Carbon Exchange 12.7 Additional Measurements 12.8 Future Experimentations References Chapter 13: Eddy Covariance Measurements over Grasslands 13.1 Historic Overview of Grassland Eddy Covariance Flux Measurements 13.2 Peculiarities of Eddy Covariance Flux Measurements over Grasslands 13.3 Estimating Grassland Carbon Sequestration from Flux Measurements 13.4 Additional Measurements 13.5 Other Greenhouse Gases References Chapter 14: Eddy Covariance Measurements over Wetlands 14.1 Introduction 14.2 Historic Overview 14.3 Ecosystem-Specific Considerations 14.4 Complementary Measurements 14.5 EC Measurements in the Wintertime 14.6 Carbon Balances and Climate Effects 14.7 Concluding Remarks References Chapter 15: Eddy Covariance Measurements over Lakes 15.1 Introduction 15.2 Existing Studies 15.3 Surface-Specific Siting Problems 15.3.1 Stratification of Lakes 15.3.2 Aqueous Chemistry of CO2 15.3.3 Land-Lake Interactions 15.3.4 Quality Control Procedures 15.3.5 Mounting Instruments References Chapter 16: Eddy Covariance Measurements Over Urban Areas 16.1 Introduction 16.1.1 Scales in Urban Climatology 16.1.2 The Urban Atmosphere 16.1.3 Exchange Processes in the Urban Atmosphere 16.1.4 Characterization of the Urban Surface–Atmosphere Interface 16.2 Conceptual Framework for Urban EC Measurements 16.2.1 Turbulence Characteristics 16.2.2 The Volume Balance Approach 16.2.2.1 Turbulent Heat Fluxes in the Context of Urban Energy Balance Studies 16.2.2.2 Evapotranspiration in the Context of Urban Water Balance Studies 16.2.2.3 CO2 Fluxes in the Context of Urban Metabolism Studies 16.2.3 Other Trace Gases and Aerosols 16.3 Challenges in the Siting of Urban EC Stations 16.4 Implications of the Peculiarities of the Urban Boundary Layer on EC Measurements 16.4.1 Advection and Storage 16.4.2 Flow Distortion 16.4.3 Night Flux Problem, Gap Filling, and QC/QA 16.4.4 Service and Maintenance of Instruments 16.5 Summary and Conclusions References Chapter 17: Database Maintenance, Data Sharing Policy, Collaboration 17.1 Data Management 17.1.1 Functions 17.1.2 Flux Tower Repositories 17.1.3 Regional Repositories 17.1.3.1 One Example: The European Eddy Covariance Flux Database System 17.1.4 The FLUXNET Initiative and Database 17.2 Data Practices 17.2.1 Contributing Data and Reporting Protocols 17.2.2 Common Naming/Units/Reporting/Versioning 17.2.2.1 Enabling Cross-site Analysis: Site Identifier, Variables, and Units 17.2.2.2 Data Releases 17.2.2.3 File Naming 17.2.3 Ancillary Data Collection 17.3 Data User Services 17.3.1 Data Products: The Example of fluxdata.org 17.3.1.1 Users and Use Cases 17.3.1.2 The Public Access Area 17.3.1.3 The Authorized User Support Area 17.3.1.4 Measurement Site Scientist Support Functions 17.4 Data Sharing and Policy of Uses 17.4.1 Data Sharing Motivation 17.4.2 Data Policy of Use 17.4.3 Additional Credit Possibilities References Symbol Index Abbreviations and Acronyms Index This highly practical handbook is an exhaustive treatment of eddy covariance measurement that will be of keen interest to scientists who are not necessarily specialists in micrometeorology. The chapters cover measuring fluxes using eddy covariance technique, from the tower installation and system dimensioning to data collection, correction and analysis. With a state-of-the-art perspective, the authors examine the latest techniques and address the most up-to-date methods for data processing and quality control. The chapters provide answers to data treatment problems including data filtering, footprint analysis, data gap filling, uncertainty evaluation, and flux separation, among others. The authors cover the application of measurement techniques in different ecosystems such forest, crops, grassland, wetland, lakes and rivers, and urban areas, highlighting peculiarities, specific practices and methods to be considered. The book also covers what to do when you have all your data, summarizing the objectives of a data base as well as using case studies of the CarboEurope and FLUXNET databases to demonstrate the way they should be maintained and managed. Policies for data use, exchange and publication are also discussed and proposed. This one compendium, is a valuable source of information on eddy covariance measurement that allows readers to make rational and relevant choices in positioning, dimensioning, installing and maintaining an eddy covariance site; collecting, treating, correcting and analyzing eddy covariance data; and scaling up eddy flux measurements to annual scale and evaluating their uncertainty.